The present invention generally relates to methods for plasma etching of metallization layers. More specifically, the present invention provides a method for anisotropic etching of an aluminiferous layer in a vacuum chamber.
In microelectronics, layers having aluminum alloys such as aluminum-silicon, aluminum-copper, or aluminum-silicon-copper are utilized in the wiring levels of integrated circuits as critical interconnect material. These layers are structured to form interconnects with a lacquer mask that is correspondingly exposed. The structuring is usually implemented in a vacuum chamber with a gas mixture containing chlorine or chlorine compounds at low pressure with high-frequency enhancement (plasma etching process). The aluminum-chlorine reaction is spontaneous and occurs without ion support. A sidewall passivation that protects the lateral aluminum sidewall against an isotropic etching attack is therefore necessary for aluminum structuring that is free of underetching, i.e. anisotropic. This sidewall passivation is constructed of lacquer constituents or the reaction products thereof that are produced by ion-assisted erosion of the lacquer mask. Such etching processes are described in, for example, the article by P. Ridley et al. in Solid State Technology, Feb. 1993, pages 47-55.
With increasing miniaturization of the structures to be manufactured, the following problems, among others, arise, given this method wherein the sidewall passivation is based on the erosion of the lacquer mask. For instance, the prescribed topography of the background (only a thin lacquer layer is present at raised locations) limits the maximum lacquer thickness that can be eroded. Moreover, the lacquer thickness is limited for exposure-orientated reasons (limited depth of field and resolution). In addition, more aluminum sidewalls per unit of area must be protected given finer and finer structures, i.e. a larger area of sidewall passivation must be produced. Still further, area relationships of lacquer to aluminum that differ greatly generally occur on a chip. Underetchings often exist due to the lack of lacquer, given insulated interconnects and correspondingly little lacquer available. Moreover, the risk of copper residues that cannot be removed is extremely high, given copper-containing aluminum alloys and a lacquer part that is locally or globally too small. Additionally, given circuits having a plurality of metallization levels for achieving higher integration densities, positively beveled aluminum sidewalls (i.e., the upper edge of the structure is not as wide as the underedge) are desirable for better deposition of the intermetallic dielectric.
The industry has adopted a number of measures to resolve the above-identified problems. For instance, increasing the lacquer erosion can improve the sidewall passivation. This improvement, however, is accompanied by a risk of unacceptable, incipient etching of raised aluminum tracks. Adding halogenated hydrocarbons can promote the formation of sidewall passivation, given coarser structures. However, this formation is no longer adequate given the finer structures. Given employment of mixtures of etching gas that lead to thinner sidewall passivation films with less of a lacquer erosion, the risk exists that gaps will arise locally in the passivation film, particularly at insulated interconnects, so that underetching occurs. Fundamentally, diminishing the height differences of the background, i.e. by global planarization, can also alleviate the problem of lacquer thickness. To this end, however, extensive modifications in the chip structure and/or in the manufacturing method are necessary. Increasing the ion bombardment can remove etching residues that contain copper; this, however, simultaneously results in an increased lacquer erosion. Finally, positively beveled aluminum sidewalls can be produced on the basis of a retreat of lacquer, accepting what is referred to as loss of dimension. Such a method, obviously, can no longer be utilized given fine structures.
Therefore, a need exists for an improved method of etching aluminiferous layers completely anisotropically.